f
different sorts.

The general structure of these bundles is more or less the same in all
grasses. A vascular bundle consists of only xylem and phloem, without
the cambium, and so no secondary thickening can take place in the stems
of grasses. Such bundles as these are called =closed vascular bundles=
to distinguish them from the dicotyledonous type of vascular bundles
which are called =open vascular bundles= on account of the existence of
the cambium.

[Illustration: Fig. 22.--Transverse section of a vascular bundle. x 250

1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. phloem or
sieve tubes; 5. sclerenchyma.]

The component parts and elements of which the vascular bundles in
grasses are composed may be learnt by studying the transverse and
longitudinal sections of these bundles in any grass. The cross and
longitudinal sections of a vascular bundle of the stem of _Pennisetum
cenchroides_, are shown in figs. 22 and 23. In the figure of the
transverse section the two large cavities indicated by the number 3 and
the two small circular cavities with thick walls lying between the
larger ones and indicated by the numbers 1 and 2 are the chief elements
of the xylem.

By looking at the longitudinal section it is obvious that these elements
are really vessels, the larger being pitted and the smaller annular and
spiral vessels. These vessels together with the numerous small
thick-walled cells lying between the pitted vessels constitute the
xylem. Just above the xylem there is a group of large and small
thin-walled cells. This is the phloem and it consists of sieve tubes and
thin-walled cells. All round the xylem and the phloem there are many
thick-walled cells. These are really fibres forming the =bundle-sheath=.
On account of this bundle-sheath the bundles are called =fibro-vascular
bundles=.

[Illustration: Fig. 23.--Longitudinal section of a vascular bundle. x
250

1. Annular vessel; 2. spiral vessel; 3. pitted vessel; 4. sieve tubes or
phloem; 5. sclerenchyma.]

[Illustration: Fig. 24.--Transverse section of a portion of the stem of
Rottboellia exaltata. x 70

1. Epidermis; 2. sclerenchyma; 3. vascular bundle.]

=Structure of the stem.=--The stem of a grass consists of a mass of
parenchymatous cells with a number of fibro-vascular bundles imbedded in
it, and it is covered externally by a protective layer of cells, the
epidermis. The stem is usually solid in all grasses in the young stage,
but as it